This invention relates to systems for driving apparatus intermittently, and more particularly to an arrangement for driving apparatus which needs now driven intermittently but reliably with a specific mechanical torque during a long period of time without maintenance.
Such drive arrangements are used especially in connection with monitoring apparatus intended to record various data such as parameters in the fields of engineering (mechanical stresses in bridges, tunnels, and other structures), utilities (flow-rates in mains and pipelines), or meteorology (air and water pollution, radiation, insolation, etc). Intermittent drive arrangements also have many other kinds of applications in the most varied fields.
One of the important requirements which have to be met by such drive arrangements is great adaptability. Due to the large variety of different uses of the apparatus, the rate of intermittency of driving (typically of driving the motor of a monitoring apparatus) might have to be one driving operation per minute, or per hour, or per day; it may also be necessary to have two driving operations per day, or one operation every two days or every week or every two weeks, or even one driving operation after a lapse of 31 days (monthly monitoring) etc. If only for the manufacture of the driving system, it is important that just one type of apparatus need be built and that the particular rates of intermittency desired can then be set by means of a very simple operation. Also for systems already in operation, it is very desirable for the person responsible for maintenance of the system to be able to change the rate of intermittency by a very simple operation and by means of equipment which is light and easily portable.
Another of the essential requirements to be met by such drive arrangements is that of long-term autonomy, for the apparatus they drive are often installed in large numbers at locations which are not easily accessible; either they are read only infrequently, or the readings are transmitted by radio to central stations. In any case, such apparatus must be capable of operating for a very long time (at least several years, even up to ten years) without requiring any maintenance. Since this equipment is often situated far from the power mains, it is fed by cells, groups of cells, or storage batteries having a large capacity and great reliability.
In order for such apparatus to operate properly, moreover, the intermittent drive arrangement must furnish a certain mechanical torque which is substantially higher than what suffices to move simple needles or pointers, for instance.
Furthermore, the requirement of adaptability, as mentioned above, is essential, first, in order to have avantageous manufacturing conditions (only one basic type to manufacture) and, second, in order to be able easily to change the rate of intermittency of apparatus already in service.
In order to achieve the aforementioned long-term reliability, one problem which must be solved, and which can be solved with present-day technology, is that of providing the apparatus with adequate, very reliable, and very long-life energy sources. In addition, however, there are two other particular problems to be solved: one relating to the energy consumption, which must be very low, meaning that high efficiency is called for; and the other relating to the need for a virtual lack of wear and tear on the various moving parts in the drive arrangement. Both of these problems have essentially to do with the type of motor used in the intermittent drive arrangement.
There is a known type of electric motor which is capable of rendering the desired service and which is distinguished by extremely low wear and tear, allowing it to operate intermittently for a number of years (up to ten years) without any maintenance. This is the stepping motor. The use of a stepping motor in a drive arrangement of the aforementioned kind therefore provides a good solution to the second of the two particular problems cited above, encoutered in satisfying the essential requirement discussed earlier. Unfortunately, the stepping motor does not furnish any solution to the first of those two problems, namely, that of very low energy consumption, i.e., high efficiency. The fact is that in a stepping motor, the electrical pulse conditions are fixed and must be such that sufficient energy is supplied to the motor at each step, taking into account the most severe conditions of moment of resistance that may be encountered. Even when the actual moment of resistance is much lower, each step of the motor uses the same amount of energy, predetermined to ensure that the motor will advance even under unfavorable conditions. Consequently, it turns out that when the mechanical power furnished (considering the necessary mechanical torque mentioned above) is compared with the actual expenditure of electrical energy, the results are far from excellent. Thus, the use of a stepping motor means that the capacity of the energy source must be considerably overdimensioned, and this is not possible without major drawbacks from the point of view of expense and space requirements.
There is another type of motor, known for its particularly high efficiency, which might be considered for use, namely, the DC motor; for besides being highly efficient, it has the advantage of automatically adapting its power consumption to the torque required. Thus, the use of a DC motor seems to solve the first of the two particular problems mentioned, but it runs into difficulties concerning the solution to the second of those problems, viz., the necessity for low wear and tear. When a small DC motor is operating continuously, it may be expected to run reliably for about 2,000 hours, or 3,000 hours for very high-quality motors. Beyond that, the motor will not operate reliably without maintenance, particularly including thorough cleaning or replacement of the commutator and brushes. Nevertheless, theoretical considerations lead to the belief that for intermittent operation, a DC-motor might be used for a number of years without maintenance. If, for example, the motor is assumed to operate for a total of two seconds per minute (e.g., about 30 ms per second), the ratio of running time to total time elapsed is 1:30; hence it might be thought that the time during which such a motor can operate without maintenance could be increased from 4 months to 120 months (i.e., 10 years of completely satisfactory autonomous running without maintenance).
However, practical testing has shown that this theory is inapplicable; DC motors operating intermittently, e.g., as described above, prove to be subject to excessive wear, making them much less reliable than what is required, after a period of approximately 10-18 months. In practice, therefore, the coefficient of multiplication of maintenance-free life owing to intermittent operation is not the hoped-for 30, but rather only 3 or 4.
When the reasons for these unfavorable results were sought it was found that the wear and tear during intermittent service does not decrease in the expected proportion because it is greatly increased, especially as concerns the commutator and the brushes, when the motor is repeatedly turned on and off. It is a well-known fact that when a DC motor starts and stops, there are current values (upon starting) and inverse voltage values (upon stopping) which are very much greater than the values during normal running and which exert a particularly harmful action upon the commutation system (commutator and brushes). Theoretically, however, these exceptionally high values should occur only during an extremely short time, so that the excess wear attributable thereto should hardly amount to more than 10%.
In the first tests carried out, as well as in the rare intermittent drive arrangements utilizing a DC motor now in existence or previously proposed, the motor was switched on and off principally by electro-mechanical switch components. Yet no matter how well such electro-mechanical switching components are designed and constructed, they have proved to exhibit a certain amount of bouncing upon actuation and deactuation, thus considerably prolonging the excess current and voltage conditions which cause undue wear on the commutator and brushes. Hence wear and tear is increased, not by some 10-20%, but by some 100-200% or even more, which explains the unfavorable results obtained during the aforementioned tests.
Reference may be made to the graphs of FIGS. 1A and 1B the accompanying drawings for an illustration of how the phenomena causing excessive wear are multiplied by switching with electromechanical means.
On the other hand, besides the above-mentioned drawback of the prior art devices, it has never been proposed to produce such driving apparatus in such a way that the rate of intermittency can be easily adapted as has been mentioned. Of course, it is known to adjust the cadence of an intermittently functioning device, but the means therefor have always been relatively complicated.